Originally Posted by T-bucket23
I would push the timing up to at least 12 BTDC. 4 is way to low. Also you should check to see what your total timing is. If you are not running a vacuum advance it should be in the 34-36 range all in by about 2800 rpm.
I would also get a lower hose with a spring in it.
After you fix the timing take it for a ride and see if it improves.
These are just the basic items, there are a lot of things that can cause overheating but you need to start with the basics. The fact that it doesnt overheat at idle does eliminate a few items.
Get back to us on the progress
Be sure and set timing! 4 deg is way too low for sbc. Will cause heating. At 4 deg you should be experiencing low power to boot.